The switch has three connectors on the rear, I’m not sure if there’s a standard reference for them but I’ve labelled them as 1,2 & 3 as shown below:
The connectors on the wiring loom of my 1987 Dolly are as follows:
When the switch is in the ‘on’ position (i.e. depressed) pins 1 & 3 are connected.
When the switch is in the ‘off’ position pins 1 & 2 are connected. (From the wiring diagram it looks like this is to provide power to the wiper parking mechanism.)